This invention relates to underwater pipeline apparatus for delivering a pig unit through a sea-bed pipeline, or for the flooding of a sea-bed pipeline with seawater.
Conventionally, pigging of a pipeline is carried out by pumping liquid or gas into one end of the pipeline in order to drive a pig train through it. This normally requires a substantial flow of liquid or gas at relatively high pressure requiring a large pump of high horsepower located on a surface vessel or surface installation. This is expensive in terms of capital equipment, cost of mobilisation to site, and the consumption of fuel. An important cost factor, where the pipeline is laid in deep water, is the need for large-bore high-pressure pipework from surface to sea-bed.
In the case of sub-sea pipeline which is initially filled with air or low-pressure gas, there already exists a hydrostatic pressure difference between the interior of the pipe and the surrounding sea. This is sometimes used to flood a pipeline under natural pressure, by simply allowing water to enter, possibly via a strainer. Our invention is a device which allows this natural pressure difference to be utilized to assist the pigging process in a controlled manner, so achieving a substantial saving.
It is not sufficient to open a valve to admit water to drive a pig or pig train through the pipeline, because some or all of the following requirements must normally be met;
a) The water admitted to the pipeline must meet certain standards for contained impurities. PA1 b) Chemicals may be required to be added to the water at a specified concentration, in order to protect the pipeline from corrosion or marine growth or to detect leaks during testing. PA1 c) The pig train's position must be recorded, normally by measuring the quantity of water admitted. PA1 d) The pig train must proceed at a speed within a specific range.
The last requirement imposes two particular problems in the context of what is proposed. Initially, the hydrostatic pressure difference is at its maximum and the pipeline could flood too rapidly if the rate were not controlled. Conversely, the head diminishes as the pipeline becomes progressively filled and will eventually fall to equal the driving pressure differential required for the pig or pig train; at this point, flooding will cease and action will be required to complete movement of the pig to the end of the pipeline.